創(chuàng)建表：

CREATE TABLE emp (

ename varchar(10) DEFAULT NULL,

hiredate date DEFAULT NULL,

sal decimal(10,2) DEFAULT NULL,

deptno int(2) DEFAULT NULL,

age int(3) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE dept (

deptno int(2) DEFAULT NULL,

deptname varchar(20) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8

表連接：當(dāng)需要同時顯示多個表中的字段時，使用表連接

內(nèi)連接：僅選出兩張表中互相匹配的記錄

外連接：既選出匹配的記錄，也選出不匹配的記錄

左連接：包含所有的左表中的記錄，即使右表中沒有和它相匹配的記錄

例如，用戶表中有部門id，使用左連接查詢右表，得到相應(yīng)的部門名稱，對于用戶表中的id在右表中找不到對應(yīng)的記錄時結(jié)果為NULL

select ename,hiredate,sal,emp.deptno,age,deptname from emp left join dept on emp.deptno=dept.deptno;

+-------+------------+------+--------+------+----------+

| ename | hiredate | sal | deptno | age | deptname |

+-------+------------+------+--------+------+----------+

| zzxl | 2000-01-01 | 2000 | 1 | NULL | dept1 |

| lisa | 2003-01-01 | 3000 | 2 | 20 | dept2 |

| dony | NULL | 2000 | 5 | NULL | dept5 |

+-------+------------+------+--------+------+----------+

右連接：包含所有的右表中的記錄，即使左表中沒有和它相匹配的記錄

select ename,emp.deptno,deptname from emp right join dept on emp.deptno=dept.deptno;

+-------+--------+----------+

| ename | deptno | deptname |

+-------+--------+----------+

| dony | 5 | dept5 |

| NULL | NULL | dept6 |

| zzxl | 1 | dept1 |

| lisa | 2 | dept2 |

+-------+--------+----------+

子查詢：進行查詢時，當(dāng)需要的條件是另外一個查詢的結(jié)果是，使用子查詢

查詢所有部門中的用戶

select * from emp where deptno in (select deptno from dept);

+-------+------------+------+--------+------+

| ename | hiredate | sal | deptno | age |

+-------+------------+------+--------+------+

| zzxl | 2000-01-01 | 2000 | 1 | NULL |

| lisa | 2003-01-01 | 3000 | 2 | 20 |

| dony | NULL | 2000 | 5 | NULL |

+-------+------------+------+--------+------+

如果子查詢結(jié)果唯一，可以用=代替in

select * from emp where deptno = (select deptno from dept limit 1);

+-------+----------+------+--------+------+

| ename | hiredate | sal | deptno | age |

+-------+----------+------+--------+------+

| dony | NULL | 2000 | 5 | NULL |

+-------+----------+------+--------+------+

注意：表連接用于優(yōu)化子查詢

記錄聯(lián)合：將多次查詢結(jié)果合并顯示

select deptno from emp union select deptno from dept;//去除重復(fù)數(shù)據(jù)

select deptno from emp union all select deptno from dept;

create table salary(userid int,salary decimal(10,2));

insert into salary values(1,1000),(2,2000),(3,3000),(4,4000),(1,null);

if判斷

select *,if(salary>2000,‘high‘,‘low‘) as statu from salary;

+--------+--------+-------+

| userid | salary | statu |

+--------+--------+-------+

| 1 | 1000 | low |

| 2 | 2000 | low |

| 3 | 3000 | high |

| 4 | 4000 | high |

| 1 | NULL | low |

+--------+--------+-------+

ifnull判斷

select ifnull(salary,0) from salary;

+------------------+

| ifnull(salary,0) |

+------------------+

| 1000 |

| 2000 |

| 3000 |

| 4000 |

| 0 |

+------------------+

case when

select case when salary<=2000 then ‘low‘ else ‘high‘ end from salary;

select case salary when 1000 then ‘low‘ when 2000 then ‘mid‘ else ‘high‘ end from salary;

總結(jié)

以上是生活随笔為你收集整理的mysql用if判断关联的表_mysql表连接，子查询以及if判断的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。

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